On a surface of a zinc-plated steel sheet for use in household appliances and building materials, white rust may be formed under a salt component-, such as sea water, containing atmosphere or a high temperature and high humid atmosphere. In this case, the appearance may be significantly deteriorated or ability for protecting a surface of iron base may be lowered. In the past, a chromate-based surface treatment agent containing chromium has been used to prevent white rust. However, regulations on chrome has been significantly intensified in view of problems of effects on environment and the human body, and in response to this, development of a chromium-free surface treatment agent has been proceeded.
As a method for treating the surface of a metallic material with a chromium-free surface treatment agent, in Patent Document 1, there disclosed a method for treating a surface with the use of a metal surface treatment agent containing a specific silane coupling agent and a phenolic resin-based polymer with a specific structure. In Patent Document 2, there disclosed a method for treating a surface with the use of a metal surface treatment agent containing a silane coupling agent, silica, and a specific metallic compound as the essential components, and further optionally comprising a thiocarbonyl group-containing compound and/or a water-soluble acrylic resin. Further, in Patent Document 3, there disclosed a method for treating a surface with the use of a metal surface treatment agent containing a specific water-soluble resin or water-based emulsion resin, a phenolic resin-based polymer with a specific structure, and a specific metallic compound.
However, the above-mentioned methods are not for forming a film imparting high corrosion resistance in place of the chromate-based surface treatment agent. In particular, corrosion resistance of scratched parts and formed parts, or corrosion resistance after degreasing with alkali, is deteriorated as compared to the chromate film. Consequently, development of a method for treating a surface having a corrosion resistance equal to or better than that of a chromate film and not containing chromium has been demanded.
Furthermore, as a method for treating the metallic material surface containing no chromium, in Patent Document 4, there disclosed a surface treatment agent for metal prepared by blending a cationic urethane resin, a cationic polycondensate of a phenolic compound, a zirconium compound, and/or a titanium compound, in an aqueous medium; a surface treatment method; and a surface treated metallic material. In Patent Document 5, there disclosed a metal surface treatment agent prepared by blending a cationic urethane resin, a cationic polycondensate of a phenolic compound, a zirconium compound, and a compound containing at least one metal selected from Li, Mg, Al, Ca, Mn, Co, Ni, Zn, Sr, W, Ce, and Mo, in an aqueous medium; a surface treatment method; and a surface treated metallic material. In addition, in Patent Document 6, there disclosed a metal surface treatment agent prepared by blending a cationic urethane resin, a cationic polycondensate of a phenolic compound, a titanium compound, and a compound containing at least one metal selected from Li, Mg, Al, Ca, Mn, Co, Ni, Zn, Sr, W, Ce, and Mo, in an aqueous medium; a surface treatment method; and a surface treated metallic material, etc.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H09-241576    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2001-31684    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2003-13252    [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2006-118012    [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2006-152435    [Patent Document 6] Japanese Unexamined Patent Application, First Publication No. 2006-152436